A standard flush valve is provided at an outlet port formed in a bottom wall of a toilet tank. When actuated it allows the water in the tank to rush out through the port and flush the toilet. Such a valve must remain open long enough for all the water to run out.
In order to prevent the tank from overfilling, there is invariably an upright overflow tube whose open upper end is at a level above which the water should not go, and whose lower end opens into the outlet port. Thus if for some reason the float-controlled fill valve fails, the tank will not overflow.
The flush valve itself normally is a mechanism provided adjacent the overflow tube so that the tank must be dimensioned to provide space for it. Furthermore this flush valve is exposed in the tank and can easily get damaged during installation or if something falls into the tank.